This invention relates to a non-fluorescing oil-based drilling fluid used in the drilling of subterranean oil and gas wells as well as other drilling fluid applications and drilling procedures. These non-fluorescing drilling fluids are composed of synthetic hydrocarbons, having molecular weights of from 120 to 1000, derived from alphaolefinic monomers and other additives such as surfactants and emulsifiers. The olefinic monomers are from the groups consisting of a carbon chain from C.sub.2 to C.sub.14 having at least one polymerizible double bond. The surfactants comprise monounsaturated fatty acids and saturated fatty acids. The emulsifiers comprise saturated fatty acids, saturated amido-amines, and saturated sulfonates. Additional materials that are common to the development and formulation of drilling fluids may also be included in the instant drilling fluids provided that the additional materials do not fluoresce in the presence of ultraviolet light. For purposes of this patent application the terms "fluoresce" or "fluorescence" will be understood to refer to a fluorescence intensity of greater than 800 when the fluid being measured is diluted with hexane at a ratio of 1 to 400.
In rotary drilling there are a variety of functions and characteristics that are expected of a drilling fluid ("drilling mud" or simply "mud"). The drilling fluid is expected to carry cuttings from beneath the bit, transport them up the annulus, and permit their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. Likewise the drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and likewise to form a thin, low permeability filter cake which seals pores and other openings and formations penetrated by the bit. Finally, the drilling fluid is used to collect and interpret information available from drill cuttings, cores and electrical logs.
Various advantages of using oil-based drilling mud in the rotary drilling of boreholes in the earth have been known for some time. In summary, it can be said that under certain conditions, one or more of the following advantages make oil-based muds more desirable than water-based muds. Lubricity characteristics are excellent. Drilling fluid weighing less than about 8 pounds per gallon can be prepared with an oil-based mud, and have proved advantageous in certain special drilling and coring situations. The penetration of the formation by water is avoided. Cores can be recovered in an in situ condition in the presence of hydrous clays and bentonites; no swelling or sloughing is experienced to reduce porosities or cause pipe sticking difficulties. There are fewer problems with hydrophilic shale formations drilled. Evaporite sections such as salt zones can be drilled with minimal leaching of the salt. Furthermore, oil-based muds can be formulated to withstand temperatures up to 500 degrees Fahrenheit.
On both offshore and inland drilling barges and rigs, drill cuttings are conveyed up the hole by a drilling fluid. With an oil-based drilling fluid, the cuttings, besides ordinarily containing moisture, are necessarily coated with an adherent film or layer of oily drilling fluid which may penetrate into the interior of each cutting. This is true despite the use of various vibrating screens, mechanical separation devices, and various chemical and washing techniques.
Mud logging is a process which involves the recording of information derived from the examination and analysis of formation cuttings. A portion of the drilling fluid is diverted through a gas-detecting device and examined under ultraviolet light to detect the presence of oil or gas. If the drilling fluid fluoresces this indicates that hydrocarbons are present and that the desired formation has been reached.
Unfortunately, in most oil-based drilling fluids either the oil used in the fluid or the surfactants that are present naturally fluoresce. This interferes with the results of the mud log because the mud logger cannot accurately ascertain whether formation hydrocarbons are present. This problem has hampered the use of fluoroscopy to determine the presence of hydrocarbon formations when either oil-based or oil-emulsion muds are used.
Graham et al., U.S. Pat. No. 2,951,940, disclose a method of detecting the presence of crude oil in the earth's strata. This method entails contacting the surface of a rock chip that has been carried to the earth's surface by a drilling mud with a reverse-wetting agent. The rock chip is then contacted with ultraviolet light. By comparing the relative fluorescence of the surface of the chip before and after the contacting operation, a positive indication of the presence of the hydrocarbons may be obtained.
U.S. Pat. No. 3,050,141 discloses a drilling fluid emulsion which comprises an oil- and water-insoluble and immiscible liquid. This liquid is characterized by the fact that it either fails to fluoresce in the presence of ultraviolet light or it fluoresces a different color from typical unrefined hydrocarbons or crude oils. This liquid forms a third layer in the presence of oil and water because it is incompatible with both oil and water.
Boyd, U.S. Pat. No. 4,787,990 discloses a non-fluorescent oil-based drilling fluid which consists essentially of branched- and cyclic-paraffins having 11 to 17 carbon atoms per molecule and further having a low aromatic content of less than about 1% and a low normal-paraffin content of less than about 5%.
Mercer et al, U.S. Pat. No. 5,096,883 teaches a hydrogenated synthetic polyalphaolefin having utility as a drilling fluid. The Mercer reference teaches various primary and secondary emulsifiers as useful for oil-based fluids. The primary emulsifiers taught by Mercer include fatty acid mixtures and modified sodium salts of higher organic acids. Secondary emulsifiers taught by Mercer include materials such as polyamides. The use of either a primary or secondary emulsifier as taught by Mercer, or a combination of primary and secondary emulsifiers, will inherently result in fluorescence characteristics greater than those permissible in the practice of this invention. In Example 1 of the Mercer patent Invermul NT (Baroid) is used as a primary emulsifier and EZ-Mul NT (Baroid) is used as a secondary emulsifier. Invermul NT is chemically equivalent to VERSAWET (M-I Drilling Fluids) and EZ-Mul NT is chemically equivalent to VERSACOAT (M-I Drilling Fluids). Referring to Table 8 of this application verifies that both VERSAWET and VERSACOAT and therefore Invermul NT and EZ-Mul NT, respectively, have fluorescence characteristics outside the scope of this invention, e.g., greater than 800. Likewise in Example 2 of the Mercer patent TRUMUL (International Drilling Fluids) and TRUPERSE (International Drilling Fluids) are identified as an emulsifier and wetting agent respectively. These materials are chemically equivalent to VERSACOAT and VERSAWET, respectively, and therefore are known to exceed the fluorescent intensity limits of this invention. Independent testing with TRUMUL (International Drilling Fluids) and TRUPERSE (International Drilling Fluids) have confirmed the fluorescent characteristics of these materials.
As can be seen from the above, the development of a drilling fluid that exhibits the desirable characteristics of an oil-based fluid and fails to fluoresce in the presence of UV light has long been an unachieved goal of the oil and gas exploration industry. With the practice of applicant's invention this goal has been realized.